Simple operation, safety, and reliability are most important requirements for measuring points for monitoring processes, be it for reasons of process optimization or for the monitoring and observance of limit values. Measuring points today supply the operator with measured value information and, in some few cases, additional information about the current condition of the system. In such case, it is monitored e.g. whether limit values are currently exceeded or whether and how sensor-specific parameters, like the glass impedance of a pH sensor, are being met. Monitoring refers, thus, to current events. In so far as a plant malfunction or shutdown causes very high costs in comparison to the price of the sensor, it is very important to exclude malfunctioning of the measuring point as far as possible in advance, in the context of which a correct and assured, measured value has special meaning.
In WO2004/025223, Wittmer et al. disclose a method for monitoring the function of a sensor, in the case of which a prognosis is given for the service life of a sensor remaining until its replacement becomes necessary. This prognosis is made on the basis of the temporal development of sensor parameters, for example periodically collected calibration data.
Independent of the question of the remaining service life, it is however also necessary to qualify the measuring accuracy of the sensor over the entire service life as certainly as possible. In order to obtain this certainty, the operator, today, must, in the case of important processes, conduct this recalibration /readjustment very frequently, even though the sensor system might, in fact, still be working absolutely correctly. Thus, especially time, but also materials, are used unnecessarily.